1. Field of the Invention
This invention relates to a method for the production of (meth)acrylic acid and/or (meth)acrylic esters. More specifically, it relates to a method for the production of (meth)acrylic acid and/or (meth)acrylic esters, characterized by removing solid substances such as polymer and precipitate which adhere to devices and pipes to be used for the production of (meth)acrylic acid and/or (meth)acrylic esters.
2. Description of the Related Art
It is well known that (meth)acrylic acid and/or (meth)acrylic esters are susceptible to polymerization and, during the process of production thereof, often generate polymers and oblige relevant devices to stop. It has been universally known to cope with this trouble by adding a polymerization inhibitor such as hydroquinone and phenothiazine to the relevant raw material prior to the production of (meth)acrylic acid and the like or performing a surface treatment on the relevant devices for the purpose of preventing solid substances such as polymer and precipitate from adhering to the devices. None of these measures, however, has attained perfect prevention of occurrence and adhesion of solid substances such as polymer and precipitate.
U.S. Pat. No. 5,728,272 discloses a method which, with a view to facilitating the removal of generated polymer, comprises stopping and washing the relevant devices while the cumulative amount of a generated polymer is small. The effect of this method in preventing the occurrence of solid substances such as polymer and precipitate, however, does not deserve to be rated fully satisfactory.
The object of the present invention is the provision of a method for inhibiting the occurrence of solid substances such as polymer and precipitate during the process for production of (meth)acrylic acid and/or (meth)acrylic esters, particularly for inhibiting the occurrence and adhesion of solid substances such as polymer or polymerized material and precipitate on the heated surfaces of a boiler and a jacket.
Further by this invention, the formation of (meth)acrylic acid due to the decomposition of an ester during the process for purification of (meth)acrylic ester can be effectively prevented.
The present inventors have found by their study that the occurrence and adhesion of such solid substances as polymer and precipitate which are formed during the process for production of (meth)acrylic acid and/or (meth)acrylic esters and the formation of (meth)acrylic acid due to the decomposition of an ester during the process of purification of (meth)acrylic esters are caused by the basic substance which emanates in a small amount from a basic solution used in washing devices and reaches the respective sites of production and purification. This invention has been perfected as a result.
Specifically, according to the invention it can provide a method for the production of (meth)acrylic acid and/or (meth)acrylic esters characterized by, when the production thereof is stopped, washing the device constructed for producing (meth)acrylic acid and/or (meth)acrylic esters with a basic solution for the purpose of removing solid substances such as polymer and precipitate formed on the surfaces of a flow way of(meth)acrylic acid and/or (meth)acrylic esters in the device during the course of the production and subsequently rinsing the device with a solvent.
According to the method of this invention, namely by washing devices, pipes, and the like with a basic solution during the process of production of (meth)acrylic acid and/or (meth)acrylic esters and thereafter rinsing them with a solvent, it is made possible to remove such solid substances as polymer and precipitate which adhere to the parts of devices and pipes in a short time and realize protracted operation of the process for the production of (meth)acrylic acid and/or (meth)acrylic esters.
Particularly in the case of the process for the production of (meth)acrylic esters, the formation of (meth)acrylic acid due to the decomposition of such esters can be effectively prevented.
The above and other objects, features, and advantages of the present invention will become clear from the following description of the preferred embodiments.
The expression xe2x80x9cprocess for the production of (meth)acrylic acid and/or (meth)acrylic estersxe2x80x9d as used herein refers to a step of cooling a (meth)acrylic acid-containing gas obtained by the catalytic vapor phase oxidation of a propylene and/or acrolein containing gas with a solvent and collecting (absorbing) the cooled gas by counter flow washing, a step of recovering (meth)acrylic acid from the collected (absorbed) mixture of the solvent and the (meth)acrylic acid (JP-B-06-15,496), and a step of esterifying the (meth)acrylic acid and recovering the resultant (meth)acrylic ester (JP-A-10-231,275). Examples of the (meth)acrylic ester may include alkyl acrylates such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, glycidyl (meth)acrylate, butyl (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.
The process which continues till recovery of the formed (meth)acrylic acid comprises devices such as an absorption column for absorbing (meth)acrylic acid from the reaction gas of (meth)acrylic acid, a stripping column, an extraction column, a solvent separation column, an azeotropic separation column in which dehydration is performed with an azeotropic solvent, an acetic acid separation column, a high boiling component separation column for removing high boiling substances such as a stabilizer, a rectification column, and a storage tank, a condenser, a re-boiler, a strainer, and a pump which are accessories of the devices annexed to the upper, middle or lower part thereof, and pipes for example between the columns, and between any of the columns and any of the accessories thereof mentioned above.
Then, the process which continues till recovery of a (meth)acrylic ester comprises devices such as a reaction column or tower for the esterification of (meth)acrylic acid with an alcohol, an ester extraction column, a more volatile component separation column, a rectification column, and a storage tank, a condenser, a re-boiler, a strainer, and a pump which are accessories of the devices annexed to the upper, middle or lower part thereof, and pipes for example between the columns, and between any of the columns and any of the accessories thereof mentioned above.
The distillation column to be used in this invention is not particularly discriminated but has only to be capable of effecting distillation. Examples of the distillation column may include a packed column, a tray column, a wetted-wall column, and a spray column. The term xe2x80x9cdistillation columnxe2x80x9d as used herein means to include accessories for a distillation column such as a heat exchanger, a condenser, a re-boiler, and a jacket. Among other items mentioned above, the device is preferred to include a distillation column and accessories of a distillation column such as a heat exchanger, a re-boiler, and a jacket.
In general, polymer and precipitate adhere to the surface of a flow way of (meth)acrylic acid and/or an (meth)acrylic ester in the state of liquid and/or vapor in the devices, accessories, and pipes between them. In the present invention, the adhered polymer and precipitate can be removed by means of a special washing in particular without overhauling the system of devices, accessories, and pipes between them constructed for the production of (meth)acrylic acid and/or an (meth)acrylic ester.
The basic solution to be used in this invention is not particularly discriminated but is only required to be a solution of a basic substance. It is preferred to be an aqueous solution of at least one member selected from the group consisting of an oxide, hydroxide, carbonate, and hydrogen carbonate of an alkali metal such as lithium, sodium, potassium, and rubidium, an oxide and hydroxide of an alkaline earth metal such as magnesium, calcium, strontium, and barium, and a mixture thereof. The basic substance is preferred to be at least one member selected from the group consisting of an oxide, hydroxide, carbonate, and hydrogen carbonates of an alkali metal and particularly preferred to be a hydroxide thereof. Among other alkali metals, sodium or potassium proves to be favorable.
Though the conditions fit for washing with a basic solution may be arbitrarily selected in view of such factors as the ease of solution of solid substances such as polymer and precipitate which adhere to the articles given to be washed, the washing temperature to be used in the bottom of a column may be proper in the range of from 50xc2x0 C. to the boiling point of the basic solution containing the polymer and precipitate peeled off, preferably from 80xc2x0 C. to the boiling point. Particularly, the alkali steaming which uses the basic solution in a boiling state proves appropriate for the purpose of washing. The washing time is not particularly discriminated. Though several hours generally suffice to produce the effect, the washing may be performed till the adhering solid substances are thoroughly peeled or dissolved. Though the pressure to be applied during the course of washing is not limited particularly, the washing is preferred to be carried out under a reduced pressure for the purpose of enhancing the flowability of the basic solution inside the device such as a distillation column. The washing performed under the reduced pressure results, for example, in increasing the effect of washing the rear side of the tray in the distillation column and ultimately curtailing the washing time. With a view to heightening the effect of the reduced pressure on the washing, the washing may be properly carried out under the pressure in the range of 100-900 hPa, preferably 150-300 hPa.
Though the mechanism of the separation or dissolution of the solid substances has not been elucidated, it may be logically explained by supposing that the ester group is saponified or the carboxyl group is converted into a salt and the resultant product is gradually dissolved in the basic solution.
When the basic solution is one member or a mixture of two or more members selected from the group consisting of aqueous solutions of an oxide, hydroxide, carbonate, and hydrogen carbonate of an alkali metal and an oxide and hydroxide of an alkaline earth metal, the concentration of the alkali metal and alkali earth metal compounds in the basic solution to be used for washing may be arbitrarily selected in respect of the ease of solution of solid substances such as polymer and precipitate which adhere to the articles being washed. It is be preferred to be in the range of 1 to 10 wt. %. Though the pressure to be applied to the site of washing is not specifically discriminated, the washing is preferred to be performed under a reduced pressure for the purpose of enhancing the flowability of the solvent within the device such as the interior of a distillation column. The washing carried out under a reduced pressure results in increasing the effect of washing the rear side of the tray in the distillation column and eventually curtailing the washing time. For the purpose of enhancing the effect of the reduced pressure on the washing, the washing may be properly carried out under a pressure in the range of 100-900 hPa, preferably 150-300 hPa.
The question whether the washing with the basic solution has been successfully performed can be confirmed by testing the distillation column for pressure loss or the heat exchanger for coefficient of heat transfer and overhauling and inspecting the device.
After the device has been washed with the basic solution, it is rinsed with a solvent. This solvent is not particularly discriminated but is only required to be capable of rinsing the detergent of an alkali metal and the like. Particularly, water is used advantageously as the solvent. Examples of the water properly used as the solvent may include deionized water, distilled water, and tap water. From the viewpoint of impurities such as sodium and calcium ions contained in water, the water to be used as the solvent may be preferred to have electric conductivity of not more than 10 mS/m, preferably not more than 1 mS/m (not including zero).
The conditions for rinsing with the solvent subsequent to the washing with the basic solution is not specifically discriminated. For the purpose of heightening the effect of washing, the washing temperature to be used in the bottom of the column may be properly in the range of 20xc2x0 C. to the boiling point, preferably 50xc2x0 C. to the boiling point, the more preferably 80xc2x0 C. to the boiling point.
The washing with the aqueous solution of a basic substance is followed by the rinsing with water. Properly, the waste water which arises from the rinsing with water may have a pH value not exceeding 9, preferably falling in the range of 6-8 at 50xc2x0 C. The waste water which emanates from the rinsing with water subsequent to the washing with the basic solution properly has a total content of the alkali metal and alkaline earth metal of not more than 50 mg/liter, preferably not more than 10 mg/liter. Particularly, it can decrease the formation of (meth)acrylic acid by decomposition of an ester thereof during the refining process of (meth)acrylate.
Further, before washing with the basic solution it can perform pre-washing with a solvent, particularly water. Especially when the amount of the polymer and precipitate to be purged by washing is large, the washing with the basic solution possibly swells the polymer to the extent of inflicting damage to the devices. This trouble may be avoided by preparatorily depriving the polymer of the part thereof which is soluble in the solvent. Though the conditions for the pre-washing is not particularly discriminated, the pre-washing temperature in the bottom of a column may fall in the range of 20xc2x0 C. to the boiling point, preferably 50xc2x0 C. to the boiling point, and more preferably 80xc2x0 C. to the boiling point for the purpose of heightening the effect of pre-washing. The pressure applied during the course of pre-washing is not particularly discriminated. The pre-washing, however, may be preferably carried out under a reduced pressure for the purpose of enhancing the flowability of the solvent inside the device such as the interior of a distillation column. The pre-washing performed under a reduced pressure results in increasing the effect of washing the rear side of the tray in the distillation column and eventually curtailing the pre-washing time. For the purpose of heightening the effect of pre-washing under a reduced pressure, it may perform the pre-washing under a pressure in the range of 100-900 hPa, preferably 150-300 hPa.